Maya LT / Cinema4D Rig to Blender?
You always have the possibility to handle the I/O using FBX. FBX can save skeletal and skinning information. Constraints and controls would have to be rebuild. Also the bone orientations and rotation orders might be different and need to be adjusted manually, depending on the import and export settings. All in all if you manage to reduce the source file to just skinned mesh, you should be able to reuse at least some of the work you have already done.